Several techniques exist in order to retrieve a fluid, such as e.g. oil or gas, from a subterranean formation. These techniques are mainly classified into primary, secondary and tertiary production methods.
Pressure is the key when collecting oil from the natural underground subterranean formations in which it forms. When a well is drilled, the pressure inside the formation pushes the oil deposits from the fissures and pores where it collects and into the wellbore where it can be recovered. Primary production methods consist in extracting the fluid using the natural flow or an artificial lift. However, the initial pressure of the oil is finite.
Secondary oil recovery is employed when the pressure inside the well drops to levels that make primary recovery no longer viable. Secondary recovery techniques involve injection of fluids or gas to increase reservoir pressure, or the use of artificial lift. However, these techniques allow only recovering around one third of the oil before the cost of producing becomes higher than the price the market would pay.
Tertiary production methods also called Enhanced Oil Recovery (EOR) may be performed on a well to increase or restore production.
EOR uses sophisticated techniques that may actually be initiated at any time during the productive life of an oil reservoir. Its purpose is not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir. Three common types of EOR operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion).
Stimulation consists of increasing permeability of the oil or gas remaining in the subterranean formation, thereby facilitating the flow of hydrocarbonaceous fluids into the well from the subterranean formation. Stimulation may be employed to start production from a reservoir when a well has initially low permeability or to further increase permeability and flow from an already existing well that has become under-productive.
One common stimulation method consists in injecting a chemical agent, e.g. an acid composition, into the subterranean formation. Such techniques, called “acidizing techniques”, may be carried out as “matrix acidizing” procedures or as “acid-fracturing” procedures.
In acid fracturing, the acidizing composition is injected within the wellbore under sufficient pressure to cause fractures to form within the subterranean formation and trigger a chemical reaction that increase the permeability of the oil within the subterranean formation. Such a fracturing requires the injection of the acid composition under high pressure, which may be complex, costly and/or inefficient.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the fracturing pressure of the formation. In this case, the permeability increase is caused primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the subterranean formation as in fracturing.
A common difficulty encountered in acidizing relates to the rapid reaction rate of the acidizing composition with those portions of the formation with which it first comes into contact. This is particularly the case in matrix acidizing. As the acidizing composition is introduced into the wellbore, the acid reacts rapidly with the material immediately adjacent to the wellbore. Thus, the acid is “spent” before it can penetrate a significant distance into the subterranean formation. For example, in matrix acidizing of a limestone formation, it is common to achieve maximum penetration with a live acid to a depth of only a few inches to a foot from the face of the wellbore. This, of course, severely limits the increase in productivity of the well.
Various methods have been attempted to reduce the reaction rate of the acid with the rock formation. For example, reaction inhibitors may be added to the acid composition. Additionally, the local temperature in the wellbore may be reduced in order to slow down the reaction rate of the acid fluid. However, all of these solutions suffer serious drawbacks by increasing the cost and complexity of the matrix acidizing operation. Therefore, it would be advantageous to have a method and a device that provides for an improved deep acid stimulation over those known heretofore.